An essential step in the production of electronic devices and circuits is the manufacture of conductive connections between the components. These connections must follow a predetermined structure in order that the componentry or the device can carry out its intended function. Such structures can be manufactured from conductive materials in a variety of manners, usually from metals or carbon (graphite). Printing processes are particularly suitable for cheap mass production, whereby the circuits are produced by depositing printing inks which contain the conductive materials onto flat electrically insulating, preferably flexible support materials in a single operational step.
When manufacturing electronic circuits, inkjet printing technology can be employed. This means that electrically conductive structures can be deposited on insulating support materials, or components which have already been deposited can be conductively connected, whereby in contrast to other printing techniques such as screen printing, a previously prepared printing mask does not have to be produced; thus, it is possible to carry out manufacturing on a small mass-production scale, or even to manufacture single parts, simply and cheaply. Such applications have been described, for example, in the article by John B Blum, “Printed Circuit Design and Manufacture”, 1 Oct. 2007.
Metal-containing or carbon-containing preparations are usually employed as printing inks; the electrically conductive material is present therein in the form of particles. For inkjet printing, such particles must have very small dimensions, usually less than 1 μm, in order to prevent the printing nozzles from becoming blocked and to prevent the conductive particles from sedimentation in the low-viscosity ink. In order to stabilize the particles against agglomeration and sedimentation, such inks also have to be supplemented with additives such as surfactants or protective colloids, for example. Inks which contain finely divided metallic silver are frequently preferred. Inks of that type are, for example, available from ANP (Advanced Nano Products) in Korea under reference DGP and DGH, from Harima Chemicals Inc, Japan under reference NPS and from Cabot Corporation, USA, under reference CCI-300. The particle size of the silver particles in those inks is in the range from 5 nm to a few hundred nm.
In addition to using rigid support materials such as glass or ceramic as the support materials to be printed, flexible films formed from polymers, in particular polyesters, are preferably employed. Following inkjet printing on such support materials, the solvent contained in the ink evaporates and the non-volatile additives as well as the silver particles remain in the printed layer. Since the additives are electrical insulators, the conductivity of such printed structures is low. For this reason, as described in the data sheets from the ink manufacturers and in the article by John B Blum, “Printed Circuit Design and Manufacture”, 1 Oct. 2007, a thermal post-treatment is necessary at temperatures of at least 100° C. to over 400° C. in order to produce a metallic conductivity in the printed structures. Particularly at low temperatures, the time required for this necessary thermal post-treatment is long, normally more than 1 hour. If higher temperatures are employed in order to reduce the treatment time, however, it is not possible to use the cheap and easily manipulatable flexible films produced from thermoplastic polymers as a support material since the stability of such foils at the high temperatures required is insufficient and they deform.
In the article “Low Temperature Chemical Post-treatment of Inkjet-Printed Nano-Particle Silver Inks” (NIP 24 and Digital Fabrication 2008 Final Program and Proceedings, page 907), Werner Zapka et al describe a process wherein the printed structures are treated with a salt solution following drying. Following use, however, that salt solution has to be removed by washing again, thereby constituting a multi-step process with subsequent repeated drying.
U.S. Pat. No. 3,652,332 A describes the use of porous support materials, in particular coated offset printing paper, to print conductive structures. The printing process used in that case is the letterpress process. The printing inks described for producing the conductive structures are carbon black or flake silver printing inks with particle sizes of distinctly more than 1 μm, which are unsuitable for the inkjet printing process. That patent teaches that the printing medium must have a certain absorbency in order to be able to produce homogeneous printing surfaces with reproducible electrical conductivity. The support materials described in the patent, preferably standard graphical papers coated with clay pigment, do not, however, satisfy the special requirements of inkjet printing with low-viscosity inks containing conductive particles, since they have a coarse and irregular pore structure and low porosity.
US 2009/0087548 A1 describes a process whereby a metallic ink is applied to a support material having a porous layer. The porous layer prevents the printed structures from spreading, so that very fine structures are obtained. In that process, the metal particles enter the porous layer; the porous layer is then removed in the subsequent heat treatment. Even though very high resolution structures can be printed using that process, a subsequent additional process step for removing the porous layer is required. In addition, that process step involves the use of high temperatures of 300° C., which means that the invention cannot be applied to the preferred flexible support materials.